Octagonal Wetting Interface Evolution of Evaporating Saline Droplets on a Micropyramid Patterned Surface

Abstract

Textured surfaces have been extensively employed to investigate the dynamics, wetting phenomena, and shape of liquid droplets. Droplet shape can be controlled via the manipulation of topographic or chemical heterogeneity of a solid surface by anchoring the three-phase line at specific sites. In this study, we demonstrate that droplet shape on a topographically patterned surface can be modified by varying the concentration of salt potassium chloride (KCl) in the droplet solution. It is found that at the beginning of evaporation the octagonal shape of the solid-liquid interface is changed to a rectangle with corners cut upon increasing the salt concentration. Such a variation in the solid-liquid interface versus the salt concentration is explained by the analysis of free energy difference. It indicates that the increases in solid-liquid and liquid-vapor surface tensions by raising the salt concentration result in a favored extension of the three-phase line intersecting the micropyramid bottom sides than the counterpart intersecting the micropyramid diagonal edges. The saline droplets experience a pinning stage at first and a depinning one afterward. The onset of depinning is delayed, and at which the instantaneous contact angle is larger upon raising the salt concentration. The three-phase line which intersects the micropyramid diagonal edges recedes ahead of the one along the micropyramid bottom sides, making the octagonal wetting interface evolve toward a circle. A close view at the droplet edge indicates that the three-phase line repeats "slow slip-rapid slip" across row by row of micropyramids during the depinning stage.

Controllable wetting and evaporation of a sessile colloidal droplet

“…A common method to measure these two angles is shown in Figure 2.5, θA is the largest static contact angle measured when replenishing liquid into the droplet and θR is the opposite, the smallest contact angle measured when removing liquid from the droplet. According to the previous experimental studies, the apparent contact angle on rough surfaces seems to be close to the advancing contact angle [36,[38][39][40]. This phenomenon may be explained by the fact that in most cases the initial spreading process of a sessile droplet on surfaces is quite similar to the advancing process when measuring θA.…”

“…As a result, when a droplet spreads on a rough surface, its total Gibbs free surface energy does not decrease smoothly. Instead, it may quickly jump from one state to another, resulting in the stick-slip manner of the droplet three phase line [36]. The chemical heterogeneous of the solid material could also induce a similar effect like the surface roughness.…”

“…They are usually arranged in a square array with the longitudinal and transverse pitches both set as b. Besides these two common pillars, some other structures are cylinder or square holes [40,66,68], stripes [40], hemispherical-top pillars [65], pyramid pillars, or cavities [36].…”

Geometrical Deposits on Microstructured Surfaces

“…An increase in ethanol concentration induced the droplet wetting area to evolve from an octagon to a square. Saline droplets have also been investigated by varying the concentration of potassium chloride (KCl) in water [30] . The wetting interface evolved from an octagon to a deformed rectangle with increasing salt concentration on micropyramid patterned surfaces.…”

Reply to Comment on “Solid–Liquid Work of Adhesion”

“…The difference is related to the way the two approaches back calculate the contact angle that should represent the work of adhesion. − Extrand developed a work of adhesion equation in a similar logic as that described in refs and , but for different geometries, and is as follows: while Tadmor et al. used the Young–Dupré equation: ,− …”